An organic electroluminescence (“electroluminescence” will be occasionally referred to as “EL”, hereinafter) device is a spontaneous light emitting device which utilizes the principle that a fluorescent substance emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied. Since an organic EL device of the laminate type driven under a low electric voltage was reported by C. W. Tang et al. of Eastman Kodak Company (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Volume 51, Pages 913, 1987), many studies have been conducted on organic EL devices using organic materials as the constituting materials. Tang et al. used a laminate structure using tris(8-hydroxyquinolinol aluminum) for the light emitting layer and a triphenyldiamine derivative for the hole transporting layer. Advantages of the laminate structure are that the efficiency of hole injection into the light emitting layer can be increased, that the efficiency of forming excited particles which are formed by blocking and recombining electrons injected from the cathode can be increased, and that excited particles formed among the light emitting layer can be enclosed. As the structure of the organic EL device, a two-layered structure having a hole transporting (injecting) layer and an electron transporting and light emitting layer and a three-layered structure having a hole transporting (injecting) layer, a light emitting layer and an electron transporting (injecting) layer are well known. To increase the efficiency of recombination of injected holes and electrons in the devices of the laminate type, the structure of the device and the process for forming the device have been studied.
As the light emitting material of the organic EL device, chelate complexes such as tris(8-quinolinolato)aluminum, coumarine derivatives, tetraphenylbutadiene derivatives, bisstyrylarylene derivatives and oxadiazole derivatives are known. It is reported that light in the visible region ranging from blue light to red light can be obtained by using these light emitting materials, and development of a device exhibiting color images is expected (refer to, for example, Patent Literatures 1 to 3 below).
Further, in late years, employing of a phosphorescent material other than the fluorescent material as the light emitting layer of the organic EL device is proposed (refer to, for example, Non-patent literatures 1 and 2 below). As described above, a great efficiency of light emission is achieved by utilizing an organic phosphorescent material excited to the singlet state and the triplet state in the light emitting layer of an organic EL device. It is considered that singlet excimers and triplet excimers are formed in relative amounts of 1:3 due to the difference in the multiplicity of spin when electrons and holes are recombined in an organic EL device. Therefore, it is expected that an efficiency of light emission 3 to 4 times as great as that of a device utilizing fluorescence alone can be achieved by utilizing a phosphorescent light emitting material.
In the organic EL devices such as those described above, constructions in which layers such as an anode, an organic light emitting layer, an electron transporting layer (a hole blocking layer), an electron injecting layer and a cathode are successively laminated are used so that light emission in the condition excited to the triplet state or from excimers in the triplet state is not quenched. In the organic light emitting layer, a host compound and the phosphorescent light emitting compound are employed (refer to, for example, Patent Literatures 4 and 5 below). In these patent literature, 4,4-N,N dicarbazole biphenyl have been used as a host compound. However, there were problems that the host compound tends to crystallize because its glass transition temperature is 110° C. or less and further, because it is too symmetrical and that short circuit or pixel defect generates in the heat resistance test of the organic EL device.
Furthermore, it was found that a crystal growth generates at the position where there is a foreign matter or a protrusion of an electrode in an occasion of vapor deposition, and that the defects generate more than the primary stage before the heat resistance test. Still further, carbazole derivatives having symmetry of order 3 are used as the host compound. However, it is too symmetrical to evade the crystal growth generation at the position where there is a foreign matter or a protrusion of an electrode in an occasion of vapor deposition, or the defects generation more than the primary stage before the heat resistance test.
Moreover, inventions about a use of a compound of a host compound and a phosphorus photoluminescent compound for an organic light emitting layer are disclosed (refer to, for example, Patent Literatures 6 to 8 below). Despite an improvement of heat resistance in Patent Literature 6, compounds disclosed in it still have favorably symmetric because they have phenylene structures composing the compounds with bonding styles coupling at almost all para positions, remaining only central benzene ring bonding at meta position, and accordingly, a problem of crystallization was unavoidable. Also, Patent Literatures 7 and 8 below disclose host materials introducing heterocyclic skeleton such as triazine skeleton in addition to carbazole skeleton, however, triazine rings bonding via phenylenes at para positions of carbazole skeleton keep high linearity of the compound, resultantly reducing triplet excitation energy of the host. Accordingly, it was difficult to convey energy from the host to the phosphorus photoluminescent dopant, and there were problems of causing degradation in efficiency of light emission particularly about blue phosphorus photoluminescent devices. Still further, although Patent Literature 9 below discloses a compound made by bonding a group having 5 or more benzene rings with carbazole skeleton, the compound is highly symmetric about its skeleton, easily crystallizable, and highly linear about the group having 5 or more benzene rings. Therefore, there was a problem that the compound reduces the triplet excitation energy.                Patent Literature 1: Japanese Unexamined Patent Application Laid-Open No. Heisei 8(1996)-239655        Patent Literature 2: Japanese Unexamined Patent Application Laid-Open No. Heisei 7(1995)-138561        Patent Literature 3: Japanese Unexamined Patent Application Laid-Open No. Heisei 3(1991)-200889        Patent Literature 4: U.S. Pat. No. 6,097,147        Patent Literature 5: International PCT Publication No. WO 01/41512        Patent Literature 6: Japanese Unexamined Patent Application Laid-Open No. 2003-31371        Patent Literature 7: Japanese Unexamined Patent Application Laid-Open No. 2002-193952        Patent Literature 8: European Patent Publication No. EP 1202608        Patent Literature 9: Japanese Unexamined Patent Application Laid-Open No. 2001-313179        Non-patent Literature 1: D. F. O'Brien and M. A. Baldo et al “Improved energy transfering electrophosphorescent devices” Applied Physics letters Vol. 74 No. 3, pp442-444, Jan. 18, 1999        Non-patent Literature 2: M. A. Baldo et al “Very high-efficiency green organic light-emitting devices based on electrophosphorescence” Applied Physics letters Vol. 75 No. 1, pp4-6, Jul. 5, 1999        